Numerical controls are used in modern machine tools to control the positioning and movement of tools relative to a workpiece. To machine a workpiece in accordance with a setpoint selection, it may be necessary to move the tool relative to the workpiece on paths established beforehand. Therefore, one also speaks of a continuous-path control. The desired paths are determined in a parts program that is executed by the numerical control. In so doing, the numerical control converts the geometrical statements of the parts program into statements for the positional control of the different axes of the machine tool.
In such a parts program, for example, any tool paths as needed are approximated by interpolation points (also known as support points), between which the continuous-path control linearly interpolates. In modern machine tools such as a 5-axis milling machine, a plurality of axes of motion are available, upon which the desired path may be projected. The stipulation for each axis of motion is then in turn made up of interpolation points (axial positions), which are approached in succession and in synchronism by each axis. This holds true both for linear axes and for angle axes.
Since a machine tool is subject to certain restrictions with respect to the maximum acceleration and also the maximum jerk (change of acceleration) in its axes of motion, it is not possible to pass through a corner, provided in the parts program, between two segments of the tool path with a finite velocity exactly, since to that end, an infinite acceleration would be necessary. Therefore, the maximum velocity with which a corner may be traversed is a function of the maximum permissible tolerance with which the actual tool path is allowed to deviate from the ideal tool path. The greater this tolerance, the higher the possible velocity. In this context, as velocity increases, a corner established in the parts program is increasingly rounded.
Similar restrictions are also true in the execution of a single path segment, for which each axis must be moved from a starting point (projection of the first interpolation point) to an end point (projection of the second interpolation point). Usually a velocity is predefined for this movement. Since, however, a sudden change in velocity at the starting point of a path segment would be associated with infinite acceleration, the velocity profile must be rounded. This rounding may be effected by filtering the velocity profile using FIR filters, as is described, for example, in European Published Patent Application No. 0 864 952. Since each path is made up of the superimposition of all axial movements, the individual path segments must be filtered such that all sudden changes in the velocity are smoothed in the same manner. Only in this manner is a synchronous velocity control or acceleration control ensured for each axis, resulting in adherence to the predefined tool path.
A disadvantage in the method of continuous-path control described is that, in machine tools having a plurality of axes, in each case the axes having the poorest dynamics (thus, for example, the lowest maximum acceleration) predefine the velocity control. Axes which are more dynamic must wait for the slowest axis involved in a path segment. Such less dynamic axes are often the angle axes of a machine tool. Moreover, in the case of a limitation by an angle axis, it itself is operated at the limit. This results in impairment of the surface quality of the machined workpiece, since in this case, the angle axis fully utilizes the path deviation allowed to it.
Therefore, an exemplary embodiment of the present invention may indicate a method for continuous-path control which supplies improved surface quality of the machined workpiece, and/or may allow shorter machining times.